A shared reference can create closeness while leaving a newcomer quietly outside it.

Every in-joke does two things in the same moment. It confirms a bond between the people who share it and draws a line around them, and nobody standing in the warm part of that circle can feel the line. The excluded person will usually smile and say nothing, because pointing it out costs more than sitting quietly outside. Belonging gets manufactured by exactly the thing that shuts others out.

When you use a reference in front of somebody new, explain it in half a sentence and move on. It costs almost nothing and changes where the edge of the group sits.